1. Field of the Invention
The present invention relates to a light receiver which receives light signals transmitted through space, and to a Fresnel lens used in the light receiver.
2. Background Art
In recent years, mobile terminals typified by mobile telephones are becoming increasingly sophisticated, allowing the user to view large volumes of contents such as moving pictures on the terminals. Such large volumes of data are conventionally transferred to terminals by wired transmission. However, wired transmission is not user-friendly partly because it requires cable connection. To overcome the disadvantages of wired transmission, wireless transmission technologies such as wireless LAN have rapidly come into widespread use. Furthermore, the next generation high-speed wireless transmission technologies such as UWB (Ultra Wide Band) using electromagnetic waves have been actively developed. Optical wireless transmission systems also have been drawing attention.
The optical wireless transmission systems have the potential of providing high-speed performance by utilizing the broad bandwidth of light and of improving communication security by utilizing such characteristics that light moves straight and is blocked by an obstacle. However, the conventional high-speed optical wireless transmission device uses an optical-axis adjusting mechanism to ensure received optical power, thereby having a lot of requirements such as the large size. On the other hand, the high-speed optical wireless transmission device must be compact so that it can be mounted on the mobile terminal such as the mobile telephone. In addition, in order to make the mobile terminal more user-friendly by not forcing the user to perform the optical axis adjustment, the light receiver used in the high-speed optical wireless transmission device is required to have a large acceptance angle. Under such circumstances, there is a need for a light receiver having sufficient light receiving characteristics when light is diagonally incident on the receiving optical system. The acceptance angle means the angle of incidence of light signals within which the light receiver can receive the light signals.
Two types of optical-axis adjusting mechanisms, mechanical type and optical type, are known to make a light receiver have a large acceptance angle. The light receiver having the mechanical optical-axis adjusting mechanism performs the optical axis adjustment by moving a lens as a component of the light receiver, or an optical component such as a light receiving element, or the light receiver itself. The light receiver having an optical optical-axis adjusting mechanism, on the other hand, is formed of a plurality of optical components such as a reflecting mirror and a condenser lens. However, the mechanical optical-axis adjusting mechanism requires a mechanical component such as a motor for driving other components and a peripheral component such as a control circuit for controlling the movements of other components. The optical optical-axis adjusting mechanism, on the other hand, requires an optical component for optical axis adjustment in addition to the lens. Thus, unfortunately, the light receivers using these conventional optical-axis adjusting mechanisms are large in size and expensive.
As a technique to realize a small optical system and a large acceptance angle, a receiving optical system using a Fresnel lens is known. FIGS. 11A to 11C show a conventional light receiver 802 using a Fresnel lens.
As shown in FIG. 11A, a light receiver 802 is constructed by a Fresnel lens 800 (hereinafter, lens 800) and a light receiving element 810. The lens 800 has lens surfaces 811 and back cut surfaces 812.
The lens 800 has a problem of lens loss due to its prismatic shape. More specifically, as shown in FIG. 11B, light signals 820b among the light signals incident on the lens surfaces 811 are collected by the lens 800, but light signals 820a are not collected because they are scattered by the back cut surfaces 812. In this case, all of the light signals 820a incident on the regions shown by bold lines 822a inside the lens surfaces 811 become scattered light and are not collected. Therefore, the regions shown by bold lines 822a are the invalid regions of the lens surfaces 811.
On the other hand, as shown in FIG. 11C, when the incident light on the lens 800 is inclined with respect to the optical axis of the lens 800, the height of the back cut surfaces 812 prevents the light signals outside the light signals 820d from being incident on the lens surfaces 811. Further, the light signals 820c are scattered by the back cut surfaces 812 and cannot be collected. In this case, the regions shown by bold lines 822b and bold lines 822c inside the lens surfaces 811 are the invalid regions of the lens surfaces 811. The configuration of the back cut surfaces 812, which is the cause of the lens loss, depends on the inclination angle of the lens surfaces 811. In other words, as the lens 800 has a shorter focal length, the lens surfaces 811 have a steeper inclination angle, thereby causing the back cut surfaces 812 to have a larger area. As a result, the lens 800 has a larger lens loss.
Thus, the conventional light receiver 802 using the lens 800 has a problem that lens loss occurs in the lens 800 itself.
A light receiver having a low lens loss of a Fresnel lens is disclosed in Patent Document 1 (Japanese Patent Unexamined Publication No. 2006-177999), which is shown in FIGS. 12A and 12B.
As shown in FIGS. 12A and 12B, a light receiver 902 is constructed by a Fresnel lens 900 (hereinafter, lens 900) and a light receiving element 910 disposed at the focal point of the lens 900. In the conventional light receiver 902, the lens 900 has lens surfaces 911 and back cut surfaces 912. The back cut surfaces 912 are inclined with respect to the center axis 913 of the lens 900. As shown in FIG. 12B, the outer lens surfaces 911 have shorter focal lengths. This allows the light beams collected on the light receiving element 910 to have a uniform spot size in the acceptance angle of the light receiver 902.
However, merely inclining the back cut surfaces 912 causes a decrease in the area of the lens surfaces 911, and hence a reduction in the light collection efficiency of the lens 900. On the other hand, if the focal lengths of the outer lens surfaces 911 are reduced to have a uniform spot size of the light beams as in the case of the light receiver 902, the lens loss increases, causing the lens 900 to have a low light collection efficiency.